(1) Technical Field
This invention relates to electronic circuitry, and more particularly to digitally selectable phase shifter and/or digitally selectable attenuator circuits.
(2) Background
Electronic phase shifter circuits are used to change the transmission phase angle of a signal, and are commonly used to phase shift radio frequency (RF) signals. Modern phase shifter circuits may be digitally controlled and thus provide a discrete set of phase states that are selected by a binary control word. Some phase shifter circuits also include a digitally controlled RF signal attenuator circuit that provides a discrete set of attenuation states that are selected by a binary control word.
FIG. 1 is a block diagram of a prior art electronic system 100 that includes a selectable phase shifter circuit 102 coupled to a selectable attenuator circuit 104 and an input/output interface 106. The selectable phase shifter circuit 102 and the selectable attenuator circuit 104 modify an RF input signal, RF_In, to generate an RF output signal, RF_Out. The illustrated embodiment may be useful, for example, in radar systems, phased array antenna systems, and cellular radio transmitters and receivers.
The input/output interface 106 allows user selection of particular phase states in the phase shifter circuit 102 and attenuation states in the attenuator circuit 104 by application of digital controls words to the respective circuits, in known manner. As one example, a 5-bit control word for phase may select one of 32 phase states for the selectable phase shifter circuit 102, and a 4-bit control word for attenuation may select one of 16 attenuation states for the selectable attenuator circuit 104. A commercial example of a similar circuit is the PE46120 Monolithic Phase and Amplitude Controller product from Peregrine Semiconductor Corporation.
Ideal phase shifter circuits provide low insertion loss and equal amplitude (or loss) in all phase states. Ideal phase shifter circuits also should operate independent of attenuation, changing only the phase of an input signal with no effect on insertion loss. However, actual phase shifter circuits have an inherent and unwanted variability of insertion loss that depends on the selected phase state. For example, FIG. 2 is a graph of one simulation of a digital phase shifter circuit showing the variability of insertion loss (in dB, measured relative to a target value of about −5.8 dB, shown by a dotted line 200) as a function of phase state selection (each state is designated by a dot on the dashed line 202). The insertion loss variability is due to internal component and path differences within a phase shifter circuit as different phase shifting circuit components are switched in or out of the signal path, as well as impedance differences between binary components and/or impedance changes in the aggregate path of the RF signal.
Similarly, ideal attenuator circuits should operate independent of phase shifts, changing only the attenuation of an input signal with no effect on phase. However, actual attenuator circuits may cause unwanted phase variations that depend on the selected attenuation state.
The above problems become particularly acute as the number of state selection bits and corresponding internal interactions increases.
Accordingly, there is a need for a phase shifter circuit that exhibits little or no effect on insertion loss when changing phase state, and a method for calibrating and selecting phases states for such a circuit. There is also a need for an attenuator circuit that exhibits little or no effect on phase when changing attenuation state, and a method for calibrating and selecting attenuation states for such a circuit. The present invention addresses these needs.